High-speed serial links are used to transmit data from rack to rack over wired media, such as a backplane. The requirements on these high-speed links are not only high data rate, for example >5 Gbit/s, but also very low power consumption, advantageously □10 mW/Gbit/s, and small chip area. The general link model is displayed in FIG. 1. A transmitter 1, integrated on-chip, gets data dnTX from a not shown processing unit, prepares the data dnTX for transmission and sends the prepared data f(t) over a transmission channel 2, which is then received by a receiver 3. For the improvement of the transmission quality the transmitter/receiver system 1, 3 and the transmission channel 2 are designed as differential data link. Therefore, the transmission channel 2 comprises two lines, wherein over the first line the data signal f(t) and over the second line the inverted data signal is transferred. Several of these transmitters 1 and receivers 3 may be integrated on-chip. The transmission channel 2 can be for example a combination of printed-circuit board, connectors, backplane wiring and cable. At high frequencies the transmission channel 2 shows attenuation, what may lead to a decreasing transmission quality. Since the attenuation of the transmission channel 2 at these data rates is substantial even for short distances, intersymbol interference (ISI) causes a significant deterioration of the jitter budget. Until the present date, the preferred signaling scheme for the data transfer is a non-return to zero (NRZ) signaling.
Assuming that the high-frequency attenuation is small enough, this transmission method according to FIG. 1 has the advantage of a relatively simple and proven implementation. Such an implementation is explained in W. J. Dally, J. Poulton, “Equalized 4 Gbps CMOS Signaling”, IEEE Micro, pp 58-56, January-February 1997. As displayed in FIG. 2, the received data g(t) is sampled in the middle of the eye 41 at the sampling point an and at the edges at the sampling point xn, where the former samples an are used to extract the data, and the latter samples xn to extract the timing information. The vertical opening VO of the eye is approximately proportional to the attenuation of the frequency at half the symbol rate, e.g. 5 GHz for a 10 Gbit/s system. If the attenuation at this frequency is too high, the vertical eye opening VO will finally become too small to assure reliable data detection. One possible solution is the use of multi-level pulse amplitude modulation (PAM), which reduces the bandwidth of the signal. This comes at the price of a decrease in the eye opening VO, for example a factor of 3 for PAM-4 as compared to NRZ, and a considerable increase in complexity. First, the transmitter 1 must be able to generate multi-level values with high linearity. Secondly, the clock recovery in the receiver 3 and the adaptive equalization require a large amount of additional hardware. Additionally, the number of edge transitions, which can be used for clock recovery is smaller in a PAM system than in a symbol rate system, which makes it more difficult for the clock recovery circuit to follow clock frequency offsets and sinusoidal jitter.
Another solution is to use partial response signaling together with a decision-feedback equalizer (DFE) at the receiver 3. A major drawback of this solution is however the problem of error-propagation, which forbids to combine such a receiver with a simple encoding scheme, e.g. a Hamming code, being able to correct single bit errors.
A further solution is to employ duobinary coding. Duobinary coding is advantageously used in transmission systems in order to reduce the transmission bandwidth or to increase the transmission capacity. In case of Duobinary coding, a special case of partial response binary coding, the system allows that a binary signaling pulse is spread over a duration of two bit times by the channel distortion. In consequence, the bandwidth requirements on the channel are reduced by roughly 35 percent. Although the pulse width sent at the transmitter side corresponds to one bit time, the duration of the received impulse is two bit times. The superposition of the received impulses then results in a pseudo-ternary signal on the receiver side, as seen in FIG. 2, where the signal at time 1.5 can take on the values of 0.85, 0, −0.85. In order to avoid error propagation when the signal is decoded, the binary signal at the transmitter has to be pre-coded with the method described in J. Proakis, “Digital Communications”, 4th edition, McGraw Hill, pp. 565-568. Advantageously, duobinary signaling allows extending the distance of binary links for only a modest increase in additional complexity, and does not suffer from error propagation.